Research on living tissue by the application of a probe to the tissue, may be difficult where the tissue must be allowed to move, as in the case of the heart and arteries of an animal. For example, in research an atherosclerosis, measurement and analysis of the oxygenation of arterial walls in living tissue can be accomplished by the insertion of a probe into the tissue. In one procedure, a probe which is small in comparison to cell size, penetrates the tissue in increments of five to ten microns. Since the radial expansion of the arteries is much larger than such increments during a heart beat, meaningful measurements require the probe to maintain a set distance from the arterial wall, within about one micron and with a phase error as close to zero as possible. It would be possible to rest a probe holder directly on the walls of the artery adjacent to the location where the probe penetrates the artery, except that the adjustable probe holder has a considerable mass which would interfere with functioning of the artery if a large portion of the holder mass were to rest on the artery. An apparatus that enabled the probe and a relatively massive probe holder to follow motions of the tissue, while applying minimal loads or other interference to the tissue, would be of considerable value.